In a ported two-stroke IC engine the piston serves to open and close the inlet and exhaust ports located circumferentially in the cylinder. One or more of these ports provide passage for a fresh charge of air or air/fuel mixture to the cylinder and one or more other ports provide exhaust of the combusted gases. The inlet ports and the exhaust ports are arranged around the cylinder so that on the downward movement of the piston the exhaust ports are uncovered first to allow the high pressure gases to escape to the atmosphere via exhaust pipes to achieve blow-down. On further downward motion of the piston the inlet ports are uncovered to allow fresh charge of pressurized air or air/fuel mixture to be delivered to the cylinder for combustion after undergoing a compression cycle. The pressurized gases also serve to force combusted gases from the cylinder, a process known as scavenging.
In conventional two-stroke IC engines, the opening of the exhaust ports and the inlet ports is symmetrical which means that the exhaust ports open first followed by the opening of the inlet ports on the downward movement of the piston, however on the upward movement of the piston the inlet ports close first followed by the closure of the exhaust ports. This action allows some of the pressurized fresh air or air/fuel mixture to escape while the exhaust port is open. This process prevents the two-stroke IC engine from being able to be supercharged or turbocharged.
Modern engines are required to be fuel efficient as well as low in exhaust emissions and for two-stroke engines to be successful in fulfilling these two criteria it is important to achieve asymmetric port timing. This means that the exhaust port must open first for the blow-down process, followed by the opening of the inlet port for the scavenging and charging process. During the upward motion of the piston, the exhaust port closes first to prevent fresh charge from escaping after the scavenging has been completed, followed by the closure of the inlet port. The period between exhaust port closure and inlet port closure is used to complete the charging cycle and, if desired, provide supercharging. It is further desirable to provide the scavenging and charging by the use of a engine driven air blower commonly known as a supercharger. In addition, this will allow the engine to have a wet sump in a similar manner to a four stroke IC engine.
There are various known forms of achieving asymmetric port timing in two-strokes. However these generally involve complex arrangements such as that described in U.S. Pat. No. 6,170,443 to Hofbrauer in which two opposed pistons operate in one cylinder with each piston phased with respect of the other such that one piston uncovers the inlet port while the other piston uncovers the exhaust port. Another opposed piston engine is the Junkers Jumo engine in which two crankshafts were used on either end of the engine and coupled together by chains or gearing slightly out of phase with each other to achieve the port asymmetry. Similar arrangements were used in the Napier Deltic and Rootes two-stroke engines. Another form used a combination of ports and poppet valves for the inlet and exhaust respectively. Other forms include the U-cylinder, the sleeve valve arrangement and opposed cam engines. In each of these alternative arrangements additional complex mechanisms contribute to additional friction losses and bulk negating the benefits of two-stroke engines.